What Can You See in 10–18 Seconds?

Most scientific endeavors seek to add to the collective understanding of a topic, whether it is the origin of a disease, the basis of human communication or the techniques that will improve the performance of an athlete, of a dairy cow or of a surveillance satellite. Ultimately, many inquiries become reduced to the exploration of the actions and interactions of molecules and atoms.

Light often comes into play in such studies, of course, and the smaller the subject, the more energetic and more tightly focused the light source must be. Only since the creation and refinement of lasers has it become possible to study molecular motion. More recently, lasers capable of generating femtosecond-long pulses of visible and near-IR light have made possible studies of atoms. They also have been used to view electrons, but only as tiny blurs that speed past too quickly to see with clarity.

Now, however, scientists are on the cusp of imaging substances down to the electron level, which they believe will open the floodgates to remarkable discoveries. The breakthrough is coming by way of reducing laser pulses to under a femtosecond long, effectively less time than it takes for one complete cycle of visible light.

In a series of articles on attosecond-scale spectroscopy, reviewers describe the fundamental physics and the technological developments that are making possible lasers that operate at a fraction of a femtosecond as well as their numerous applications.

Philip H. Bucksbaum of Stanford University in California introduces the concept of attoscience, the “study of physical processes that occur in less than a fraction of a cycle of visible light.” He notes that the basis of all chemistry, for example, is the interplay between atomic nuclei and electrons in motion, and that moments exist within chemical reactions when the atoms are in transition-state configurations that can be observed only at time scales less than 1 fs.

He describes the technology for producing and measuring attosecond light pulses as well as the key concept of high harmonic generation, which occurs when electrons are pulled away from an atom at the beginning of a light cycle but then released to return to their original state later in the cycle, losing some energy as x-ray photons in the process.

Eleftherios Goulielmakis of Max Planck Institut für Quantenoptik in Garching, Germany, and his colleagues discuss what they dub lightwave electronics, using controlled light waves — as from highly intense lasers — to steer electrons inside and around atoms. They review the developments in optical technology that have led to attosecond-pulse lasing and note that the nascent technique could lead to novel streak imagers and to supercontinua of coherent light. They also observe that the technology soon will be compact and affordable enough to become widely available even to smaller laboratories.

Lastly, Henry C. Kapteyn and his colleagues at JILA and the National Science Foundation Center for Extreme Ultraviolet Science and Technology at the University of Colorado at Boulder write that attosecond-scale physics will make possible tabletop sources of very low wavelength coherent x-rays. They trace the events leading to the birth of attosecond science and describe its possible applications.Science, Aug. 10, 2007, pp. 765-778.